1. Field of the Disclosure
This disclosure relates to a method for fabricating low density, composite thermoelectric materials whose internal micro-structure creates insulation materials with energy generating properties by producing a significant reduction in the net thermal conductivity through the composite type thermoelectric material without a corresponding but opposite effect on the electrical properties, and a unique method for using silicone resins and metal bearing compounds for in-situ formation of thermoelectric materials within these composite thermoelectric material structures.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in the background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present invention.
Thermoelectric materials can be used for both power generation or cooling. A parameter called the Figure of Merit is usually used to define the performance of a thermoelectric material, and the Figure of Merit is a function of the Seebeck coefficient, which is a measure of the magnitude of the induced thermoelectric voltage in response to a temperature difference across that material, and the electrical conductivity and the thermal conductivity. A high Seebeck coefficient and a high electrical conductivity improve the Figure of Merit, while a high thermal conductivity decreases its value. Currently available thermoelectric materials for both Thermoelectric generators and Thermoelectric cooling use high density solid blocks of the thermoelectric materials in their construction and therefore the thermal conductivity of the solid material is difficult to reduce. Considerable work has been done on many different thermoelectric materials trying to minimize the thermal conductivity value of the high density solid material including replacing a small percentage of the atoms within the material with alternatives, using embedded nano structures and layers, and aligning the crystal orientation.
Almost 60% of the energy from the gasoline used by an automobile is lost and goes out the tailpipe as heat. If even a few percent of this “wasted heat” or thermal energy could be converted into electrical energy, then the economic benefit would be huge, reducing fuel consumption, total greenhouse gas emissions per mile driven, and in the case of the military, actually saving soldiers' lives, where many injuries and deaths occur while protecting fuel convoys. Large quantities of waste heat are available from all transportation sources, consumer goods, and industrial processes. As recognized by the present inventor, waste heat that could potentially be reused comes from fossil fuels, the burning of wood and vegetation, and even excess solar energy with examples being the high temperatures in a household attic to the large amount of unused thermal energy generated in concentrated solar collectors.
The best thermoelectric materials made today are made from high density solid, semiconductor type materials with examples being bismuth telluride, lead telluride, clathrates, Skutterudites, silicon-germanium, silicides, and tetrahedrites, and even the best materials have a relatively low efficiency in the conversion of heat into electrical energy. They can convert only a small percent of the waste thermal energy to electrical energy, and some of the best materials also use rare earth elements, which are easily subject to export limits, price manipulations and scarcity. Fabrication processes for even small high performance modules are expensive and the manufacture of thermoelectric materials that can cover large areas from a few square feet to hundreds of square feet is cost prohibitive
Most commercially available thermoelectric materials made today for power generation or cooling are semiconductor based materials, which are produced as solid to near solid blocks from pressed or by sintered powder or crystallization, and because they are high density solids, the thermal conductivity through the material is relatively high. A highly limiting factor in the efficiency and performance of a thermoelectric material is the thermal conductivity through the material, and there is extensive work being done to reduce the thermal conductivity in essentially solid materials using replacement atoms within a polycrystalline structure, and the incorporation of nanolayers and nano-materials into the thermoelectric elements.
FIG. 1 is an illustrative view of a typical conventional, thermoelectric couple 25 being used as a thermoelectric generator with a heat source 1, a high density, solid n-type element 2, a high density, solid p-type element 3, a substrate of non-electrically conducting material 4 at the hot side, and an electrically conducting strip 5 connecting the high density, solid n-type element 2 and the high density, solid p-type element 3 together on the hot side of the system.
On the opposing, or cold side 9, of the high density, solid n-type element 2, an electrically conducting strip 6 is attached. A separate electrically conducting strip 10 is attached to the high density, solid p-type element 3. The conducting strip 6 and the conducting strip 10 are isolated from the cold side 9 or heat sink material and each other by the use of a non-electrically conducting substrate material 11 on the cold side 9.
The temperature difference from the heat source 1 to the cold side 9 causes heat to flow through the system in the direction 12 shown from the heat source 1 to the cold side 9.
The high density, solid n-type elements 2 are commonly composed of materials, which are modified or doped to create an excess of free electrons 7. The high density, solid p-type elements 3 are commonly composed of materials, which are modified or doped to create excess holes 8. These semiconductor elements are commonly composed of high density, near solid Bismuth Telluride, Lead Telluride, Iron Silicide, or Silicon-Germanium or other materials. The excess electrons 7 and excess holes 8 operate as charge carriers and energy carriers. Ceramic layers or plates of the substrate of non-electrically conducting material 4 and non-electrically conducting substrate material 11 are commonly used as substrates to strengthen/support the thermoelectric structure and to provide electrical insulation for the high density, solid n-type element 2 and high density, solid p-type element 3, semiconductor elements from the outer hot side and cold side materials.
The temperature difference from the heat source 1 to the cold side 9 causes the excess of free electrons 7 and excess holes 8 to migrate from the heat source 1 to the cold side 9, and when the conducting strip—6 and the conducting strip 10 at the cold are electrically connected together through an external load, a current will flow in the direction of 12. The high density, solid n-type element 2 and the high density, solid p-type element 3 thermoelectric semiconductors are often connected electrically in series and thermally in parallel to make a thermoelectric generator.
Electrically connecting two materials of different compositions such as the high density, solid p-type element 3 and the high density, solid n-type elements 2 as shown in FIG. 1 can provide a current flow at a voltage and generate electricity when a temperature gradient across the material is maintained. This is known as the Seebeck Effect, which is the conversion of temperature difference directly into electricity. These same elements can be used in reverse as a Peltier cooler where electricity running through the material causes the material to become hot on one side and cold on the other.
Most commercially available p-type and n-type elements for thermoelectric generators and Peltier coolers are fabricated by the mixing of fine milled powders comprising exact mixtures of the fundamental elements of the desired thermoelectric material. These mixtures are then heated at the appropriate temperatures, and processed using various crystal growth and/or sintering techniques needed to form the desired solid to near solid thermoelectric material structure. Through the thickness thermal conductivity can be moderately lowered by complex layering and doping, but the thermal conductivity is still relatively high and the density of the final processed thermoelectric element is high, generally in the range from four grams per cubic centimeter (4 g/cm3) to eight grams per cubic centimeter (8 g/cm3).